The present invention relates to optical true-time delay generation. More specifically, the present invention relates to true-time delay generation for multiple aperture optical beam steering devices.
Optical beam steering is often required where information from an optical beam must be relayed from one location to another. High data rate, secure laser communications, target designation, and laser radar are a few of the applications in which optical beam steering is required. Optical beam steering may be provided by a single aperture, which directs a light beam in the desired direction.
Devices for single aperture beam steering are well known in the art. Single aperture beam steering can be implemented with electromechanical systems. Such systems generally consist of a mirror mounted on an electrical actuator. These systems provide relatively low losses for the strength of the reflected beam. However, such electromechanical systems are limited to low response frequencies up to the order of 1 KHz. The moving parts of an electromechanical system along with size and weight factors are considered to be major limitations of such a system.
Multiple aperture beam steering devices may be provided by compact arrays of non-mechanical beam deflectors, such as optical micro electromechanical system arrays (O-MEMS) or liquid crystal arrays. The optical signal provided to these devices is generally split into multiple optical signals. The array then actually consists of multiple optical apertures which act to steer and radiate multiple optical signals in a desired direction. However, the application of multiple aperture steering devices for fast optical communications and radar application require precise synchronization of the optical signals at the individual apertures for different pointing angles of the device. This is necessary to avoid signal scrambling due to mixing of non-synchronized outputs emerging from individual emitters.
Synchronization is required since the multiple apertures are generally deployed in a relatively flat plane. Thus, when an optical signal is steered to an angle other than exactly perpendicular to that plane, unsynchronized outputs from the individual apertures do not arrive at a receive point at the same time. This problem is particularly seen when the optical signal comprises pulsed signals. In this case, the optical pulse received from the radiating element furthest from the receive point will lag the pulse received from the closest radiating element. Performance of the optical transmitting system is improved when the individual optical beams are made time-coincident to create a time-coincident optical beam.
Applying a time delay to each optical signal before it is radiated provides the capability to generate an optical beam that is synchronized at a receive point. Controlling the delay of signals from individual transmitting elements is the principle behind beamsteered radio frequency phased array antenna systems well known in the art. Phased array antenna systems employ a plurality of individual antenna elements that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. In a phased array, the relative phases of the signals provided to the individual elements of the array are controlled to produce an equiphase beam front in the desired pointing direction of the antenna beam. The premise of a true-time delay beamsteered phased array is to steer the array beam by introducing known time delays into the signals transmitted by the individual antenna elements. Accurate beam steering of a pulsed optical beam from individual optical elements similarly requires introducing time delays into the optical signals from individual optical elements to produce an equiphase optical beam front.
True-time delay for optical signal transmission may be achieved by purely electronic means by splitting an information carrying electronics signal into a number of channels equal to the number of optical apertures. The delay required for optical beam steering is then applied to each one of the channels separately. The properly delayed signals then drive electro-optic modulators that control the optical outputs of the corresponding apertures. This electronic approach to true-time delay for optical beam steering requires very sophisticated and fast electronics that increase the cost and complexity of the system.
Optical control systems for producing selected time delays in signals for phased array antennas are well known in the art. Different types of optical architectures have been proposed to process optical signals to generate selected delays, such as routing the optical signals through optical fiber segments of different lengths; using deformable mirrors to physically change the distance light travels along a reflected path before transmission; and utilizing free space propagation based delay lines, the architecture of which typically incorporates polarizing beam splitters and prisms. These techniques may also be used for providing the true-time delays required for optical beam steering. However, these techniques are also costly and complex.
A true time delay feeder for phased array antenna has been proposed by Corral et al. in xe2x80x9cContinuously Variable True Time Delay Feeder for Phased Array Antenna Employing Chirped Fiber Gratingsxe2x80x9d, IEEE Trans. Microwave Theory and Tech., vol. 45(8), 1997, p.1531, and in xe2x80x9cTrue Time-Delay Scheme For Feeding Optically Controlled Phased-Array Antennas Using Chirped Fiber Gratingsxe2x80x9d, IEEE Phot. Tech. Lett., vol 9(11), 1997, p. 1529. In the system described by Corral et al., each element of the antenna is fed by an individually-tunable optical carrier modulated by the microwave signal. The carrier passes through a dispersive element, a chirped fiber grating, which introduces a delay. The delay for each antenna element is controlled by tuning the corresponding optical carrier. As indicated above, this technique for RF phased arrays can be applied for a multiple aperture optical beam steerer. The true time delay feeder described by Corral et al, however, requires a large number of independently tunable sources (equal to the number of elements in the array). Moreover, for some applications, just as many modulators may be required. Thus, an optical beam steerer according to the teachings of Corral et al. amounts to a complicated and cumbersome system.
Hence, a need exists in the art for a true-time delay generator for multiple aperture optical beam steering. Thus, it is desirable to provide a method and apparatus for optical true-time delay generation that is relatively simple and easy to implement and assemble. In addition, it is desirable to provide such a true-time delay generator through the use of off-the-shelf parts and technologies.
It is therefore an object of the present invention to provide a method and apparatus for providing a true-time delay generator for multiple aperture optical beam steering. It is a further object of the present invention to provide a true-time delay generator that is simple and easy to assemble. It is still a further object of the present invention to provide a true-time delay generator that can be provided through the use of off-the-shelf technologies and parts.
Pursuant to the present invention, a true-time delay generator is provided by a single spectrum broadening element, a single dispersive element, and a plurality of tunable optical filters. The single spectrum broadening element and dispersive element allow a single laser source and modulator to be used to drive the true-time delay generator. The tunable filters are individually controlled such that the outputs of the filters comprise optical pulses with different delays. The pulses may be directed to a multiple aperture optical system, which transmit the pulses in a specified direction. Synchronization of the transmitted pulses at a receive point is achieved by controlling the delay applied to the pulses for each individual aperture.
A method of optical true-time delay generation according to the present invention is provided by the steps of: providing a stream of optical pulses; broadening each pulse in the stream in the spectral domain, preferably to a near top hat shaped spectrum; dispersing each pulse in the stream in the time domain; splitting the stream of pulses into several streams of pulses; and filtering each stream so as to select a different spectral component in each stream. Selection of different spectral components provides streams of pulses which have different delays with respect to each other. Control of the filtering provides control over the delays for each stream. The delayed streams of pulses may be amplified and radiated by a multiple-aperture optical radiator, or may be converted to radio frequency pulses for radiation by a multiple-aperture antenna.